1. Field
The various circuit embodiments described herein relate in general to circuits and methods for battery power-path management, and, more specifically, to circuits and methods of the type described for monitoring, charging, and protecting rechargeable batteries across a wide range of operating voltages and conditions, including low battery voltage conditions.
2. Background
A rechargeable battery pack is a critical block for many electronic products, such as personal computers, camcorders, digital cameras, cell phones, handheld power tools, and the like. Due to its high capacity, the battery pack needs to be monitored and protected against various fault conditions that could lead to catastrophic failure of the battery. Battery power-path management is a critical block for providing such protection functions.
A typical power-path management circuit 10 is shown in FIG. 1 to which reference is first made. The power-path management circuit 10 is used in conjunction with a rechargeable battery 12, which includes one or more battery cells, two battery cells 14 and 16 being shown for illustration, connected between the BAT terminal and ground 22. The charger voltage is connected between the PACKP and PACKN terminals 24 and 26.
Traditional power-path management uses a diode-OR function of the PACKP and BAT voltages, “PACKP” referring to the charger voltage and “BAT” referring to the battery voltage of the battery being recharged. Thus, a pair of diodes 30 and 32 are connected between the BAT terminal 20 and the PACKP terminal 24, with their cathodes connected at node 33 at which the output voltage from the circuit 10 is derived.
A pair of MOSFET transistors 34 and 36 is also connected between the BAT terminal 20 and the PACKP terminal 24, the gates of which are controlled by drivers (not shown) in the monitoring, protection, and control block 40. The MOSFETs 34 and 36 are used generally for protecting the battery 12 from fault conditions, for example, an overvoltage of a possible bad charger. Thus, the MOSFETs 34 and 36 are controlled to be off when over-current, over-voltage or under-voltage faults occur.
Finally, a sense resistor 44 is connected between the PACKN terminal 26 and the ground terminal 22. Typically, the sense resistor 44 and the MOSFET transistors 34 and 36 are relatively large components, and are provided separately from the circuit 46 containing the monitoring, protection, and control block 40 and the diodes 30 and 32, for example, on a printed circuit board (not shown), or the like, associated with the battery pack with which the circuitry is used.
In operation, if the charger voltage at the PACKP terminal 24 is higher than the battery voltage at the BAT terminal 20, then PACKP-Vd is used as the output voltage on node 33 (Vd being the voltage drop across one of the diodes 30 or 32). On the other hand, if the charger voltage is removed, and the voltage at the BAT terminal 20 is higher than the voltage at the PACKP terminal 24, then BAT-Vd is used as the output voltage on node 33.
However, for applications that require low battery voltage such as 1.8V, the diode voltage drop, Vd, (normally around 0.6V) is too big, since the circuits operating from the voltage on node 33 will need at least 1.8V to operate correctly. One way to lower the minimal operating voltage is to directly connect the battery to the monitoring, protection, and control block 40′ as shown in the circuit 10′ in FIG. 2, to which reference is now additionally made. In this circuit arrangement, if the battery voltage is merely low, it will be charged up from the PACKP node by a charger, but when the battery is deeply-discharged, an instantaneous system power-up is generally not possible because the circuits of the internal monitoring, protection, and control block 40′ are not operational. That is, the battery voltage has to be high enough to activate the monitoring, protection, and control block 40′. In addition, some functions, such as protecting the battery when it is too low, are hard to implement. Indeed, some applications require that the system be powered-up by the charger when the battery is deeply discharged.
Other power-path management techniques have been advanced, for example, in integrated circuit charger systems. For instance, a circuit 50 in FIG. 3, to which reference is now additionally made, shows one example that controls PMOS gate and backgate terminal voltages in order to regulate charger outputs. The circuit includes PMOS transistors 52 and 54 in the power-path between the AC and USB inputs 56 and 58 and the output node 60. Still, a diode-OR circuit formed of diodes 62 and 64 is connected between the AC and USB inputs 56 and 58, with their cathodes connected to a bandgap voltage regulator 66. The bandgap voltage regulator 66 provides a regulated output on line 68, for example, of 2.5V, which serves as a reference voltage which is compared to the output voltage on output node 60 by operational amplifiers 70 and 72 to control the respective gates of PMOS transistors 52 and 54. The diodes 62 and 64 again introduce a diode drop, Vd, thereby limiting the low voltage operation of the circuit.
The voltage on the backgates of the PMOS transistors 52 and 54 are controlled by comparator circuits 76 and 78. The comparator circuit 76 includes a pair of PMOS transistors 80 and 82 connected between the AC input 56 and the output node 60. A comparator 84 is also connected between the AC input 56 and the output node 60 to control the backgate of PMOS transistor 52, as explained more fully below.
Similarly, the comparator circuit 78 includes a pair of PMOS transistors 86 and 88 connected between the output node 60 and the USB input 58. A comparator 90 is also connected between the output node 60 and the USB input 58 to control the backgate of PMOS transistor 54, as explained more fully below.
In operation, the comparator 84 compares voltages on the AC input terminal 56 and the output node 60 to decide if the input voltage at AC is greater than the output voltage, OUT. The comparator 84 is configured so that if the input voltage at AC is greater than the output voltage, OUT, then ACH=1 and ACHZ=0. This connects the backgate of PMOS transistor 52 to the AC input terminal 56, and powers the operational amplifier 70 to regulate the voltage on the output node 60 to be some programmed value (for example, 4.2V).
Similarly, if the USB input 58 is selected, then the comparator 90 compares the voltage on the USB input 58 with the voltage on the output node 60 to decide if the input voltage at USB is greater than the output voltage, OUT. The comparator 90 is configured so that if the input voltage at USB is greater than the output voltage, OUT, then USBH=1 and USBHZ=0, this connects the backgate of PMOS transistor 54 to the USB input 58 and powers the operational amplifier 72 to regulate the output voltage on node 60 to be some programmed value (again, for example, 4.2 V).
This architecture works well for integrated circuit charger systems, but it is not directly useful in battery monitoring systems, for several reasons. First, the voltage regulation of the circuit 50 needs a reference voltage, VBG, from the bandgap voltage regulator 66, which has to be powered from diode-or of the AC and USB inputs 56 and 58. This requires the voltages on the AC and USB inputs 56 and 58 to have enough headroom for the bandgap. Although it is good for charger applications where the minimum voltages on the AC or USB inputs 56 and 58 are higher than 4.3V, input power supplies for battery monitoring solutions do not always meet that. More and more applications in battery monitoring solutions area require power supply voltages of at least 2V, or so, to support new battery systems.
Secondly, the amplifiers and the bandgap circuits may consume some power that is appropriate for charger applications but not acceptable for battery monitoring applications. Battery monitoring systems tend to have more stringent power consumption requirements which are considered as overhead. As long as the voltage on the AC or USB inputs 56 and 58 are in a normal range (for example, greater than 4.3V), the operational amplifiers and bandgap circuits are consuming power for the regulation.
What is needed is power-path management circuits and methods that support precharge functions for a battery with as low as 0 volts, that support normal operation, even if the battery is as low as 0 volts, that provide a proper power-path during unexpected events such as short-circuit in discharge, come-and-go keychain short or brown-out events, over-current in charge, and over-current in discharge, that provide normal fast charge and normal discharge functions, and that are suitable for use in battery monitoring solutions.